Liquid level indicator

ABSTRACT

A liquid level indicator for a steam boiler or the like having a differential pressure unit (DPU) connected from the top and the bottom of the boiler. A static pressure sensor is also connected from the top of the boiler. A pointer is rotatably mounted on a base contiguous to a liquid level scale. The pointer is driven by the output of a series of linkages having inputs from both the DPU and the static pressure sensor. The static pressure sensor is employed to produce a correction because the density of boiling water is different at different temperatures and pressures. Without the correction, the indicator would indicate low at elevated pressure on full tank conditions and would indicate high on empty tank conditions. Both indications are hazardous because turbine damage, tube burn out and explosions are possible. The pointer has a fixed pivot for improved accuracy.

imited Stes Pate [1 1 Lawford et al.

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[451 July to, 11973 LIQUID LEVEL INDICATOR [75] Inventors: Victor Nicholas Lawford, Pasadena;

. Richard Paul Granada, West Covina, both of Calif.

{73] Assignee: international Telephone and Telegraph Corporation, New York, NY.

[22] Filed: Nov. 12, 3971 211 Appl. No.: 198,209

[52] US. Cl. 73/299, 73/432 A [51] Int. Cl. Gilli 23/16 [58] Field of Search 73/299, 438, 432 A {56] References Cited UNITED STATES PATENTS 3,371,534 3/l968 Akeley 73 299 2,791,906 5/l957 Vetter l. 73/299 Thomas E.

l 5 7 ABSTRACT A liquid level indicator for a steam boiler or the like having a differential pressure unit (DPU) connected from the top and the bottom of the boiler. A static pressure sensor is also connected from the top of the boiler. A pointer is rotatably mounted on a base contiguous to a liquid level scale. The pointer is driven by the output of a series of linkages having inputs from both the DPU and the static pressure sensor. The static pressure sensor is employed to produce a correction because the density of boiling water is different at different temperatures and pressures. Without the correction, the indicator would indicate low at elevated pressure on full tank conditions and would indicate high on empty tank conditions. Both indications are hazardous because turbine damage, tube burn out and explosions are possible. The pointer has a fixed pivot for improved accuracy.

5 Claims, 7 Drawing Figures Patented July 10, 1973 2 Sheets-Sheet l /0 N 1o 20 5o ll LiQ UllE LEVEL flNDlCATOlR BACKGROUND OF THE INVENTION This invention relates to liquid level indication apparatus, and more particularly to accurate means for indicating the level of a liquid in a receptacle which is maintained at varying temperatures and pressures.

ln the past, steam boilers have required the use of apparatus to indicate the level of water therein. However, the density of water changes with temperature. It is also a function of boiler pressure if the temperature is the boiling point of the water at the particular pressure.

A liquid level indication error has existed in such prior art apparatus because of the said change in water density. For this reason a correction was made as a function of the boiler pressure. However, the indicator pointer, in this case, required the use of a movable pivot with a fixed indicator scale. This therefore introduced other errors.

SUMMARY OF THE INVENTION in accordance with the device of the present invention the above-described and other disadvantages of the prior art are overcome by providing a fixed indicator pointer pivot with a fixed scale in which the pointer is driven both by the difference between the pressures at the top and bottom of the tank, and also by the pressure at the top of the tank.

The above-described and other advantages of the invention will be better understood from the following description when considered in connection with the accompanying drawings.

BRlEF DESCRlPTlON OF THE DRAWINGS in the drawings, which are to be regarded as merely illustrative:

FIG. l. is a front elevational view of one embodiment of the present invention;

FlG. 2 is a sectional view of a portion of the apparatus taken on the line 2-2 shown in FIG. ll;

PEG. 3 is a sectional view of a scale support taken on the line 3-3 shown in FIG. ll;

FIG. 4 is a perspective view of a static pressure sensor which is hidden by the scale support in FIG. 1;

HG. 5 is a line diagram of the linkages in the indicator movement.

EKG. 6 is a graph indicating the change in density of water and steam with temperature and pressure; and

FIG. '7 is a graph indicating the change in density of water with the performance of the liquid level indicator of the present invention.

DESCRIPTlON OF THE PREFERRED EMBODIMENT in FIG. l, a differential pressure unit lltl is mounted in a fixed position relative to a base plate llil. Unit litl has one inlet l2 for connection to the top of a boiler. Unit 10 also has an inlet 13 for connection to the bottom of the boiler. Plate lli has a hole M therethrough through which a boss l5 of unit it) projects.

Unit 10 has an output shaft 116 rotatable relative to boss l5. Shaft 16 rotates through an angle which is directly proportional to the difference between the pressures in inlets l2 and 13.

Unit 10 may be entirely conventional. That is, all of its components parts may be conventional, if desired. This includes but is not limited to inlets l2 and 113, boss 25 and shaft l6.

An arm 17 is fixed to shaft .36. A. linkage i3 is pinned to shaft 17 at E9. Linkage i8 is also pinned to an arm 20 at 2E. Arm 20 is fixed relative to an arm 22. Both arms 20 and 22 are fixed relative to a shaft 23. A linkage 24 is pinned to arm 22 at 25. A linkage 26 is pinned to linkage 24% at 27. An arm 28 is fixed relative to a pointer 29. Both arm 2% and pointer 29 are fixed relative to a shaft Bill which is not movable axially or in a direction perpendicular to its axis, but is rotatable with arm 2% and pointer 29. Shaft 30 is rotatably mounted on plate ill.

Linkage 26 is pinned to arm 28 at 311. Pointer 29 moves over a scale support 32 which carries a scale 33 calibrated, for example, in inches of water.

In general, the scale 33 of this type of instrument shows whether or not the boiler contains a water level above or below a predetermined desired water level. Thus, when pointer 29 is vertical at the center of scale 33, the boiler has been filled to the said predetermined level. When the pointer 29 moves to the left of center, the boiler water level is low. When the pointer 2E3 moves to the right of center, the boiler water level is high.

A plate 341 is fixed relative to plate 11 in a position spaced therefrom by three pins 35. Pins 35 are fixed both to plate ill and to plate 34.

Plate 34 has a short shaft 36 fixed thereto. A disc 37 is rotatable on a plate 34 about a shaft 36. Disc 37 is rotatable by movement of a linkage 38. Linkage 38 is moved more or less vertically up and down by a mechanism to be described. The upper end of linkage 38 is pinned to an car 39 of disc 37 at 4t A linkage 4lll is pinned at :2 to an ear 43 of disc 37. Linkage lll is also pinned at 44 to linkage 24.

The spacings of some of the parts relative to plate l l are shown in H6. 2.

Scale support 32 is fixed relative to plate llll as shown in FIG. 3. I

As shown in FlG. t, a helical Bourdon 45 has its lower end fixed relative to plate lit and is provided with a pressure inlet as for connection to the top of the boiler. Bourdon tube 45 may be entirely conventional. At its upper free end, an arm 47 is fixed thereto. Another arm tltl is fixed to a Z-shape rod 49 and to a pin 50 which is also fixed to the arm 47. A member 511 is also fixed to rod 49. The lower end of linkage 38 as viewed in FIG. ii is pinned at 52 to member 5i.

In EEG. 5, for clarity, a few of the linkages have been illustrated as single lines. in effect, the linkages 2 and 26 form a single variable linkage indicated by the dotted line at 53. The length of the dotted line 53 is thus dependent upon the angular position of disc 37.

OPERATION lFrom EEG. ll, l and 5, it will be appreciated that movement of shaft lt to the right will cause rotation of pointer 29 to the right. if the water level remains constant, this will indicate a decline in differential pressure and, most probably, a decline in static pressure. A decline in static pressure will cause movement of linkage 38 in a direction of arrow 5 in ElG. i. This will cause disc 37 to turn in the direction of arrow 55. As a result, the linkage indicated by dotted line 53 in FlG. 5 will shorten, and the movement of linkage 38 in direction 54 will cause pointer 29 to move to the right, upscale. in other words, where the water level is constant, and a change in differential pressure causes the instrument to read to low, the Bourdon tube 45 adds a correction to life point 29 to a higher reading. Conversely, when the water level remains constant, and the differential pressure increases, the output to Bourdon tube 45 tends to hold pointer 29 at a constant reading.

When water is heated to boiling in a boiler, its density changes as follows:

Pressure Temperature Water Steam PSIA F. Lbs./Cu.Ft. lbs./cu.ft. Reference 60.0 62.4

In accordance with the foregoing, the changing densities would normally result in a liquid level gage, hydrstatic type, reading low at high water levels and indicating high when the boiler is empty or near empty of water. This is illustrated in FIG. 6. Note that not only does water decrease in density, but steam also increases in density.

The device of the present invention overcomes this problem as indicated in FIG. 7. Curve 56 is the characteristic curve of the invention when it is calibrated to produce zero output error at point 57 where the tank is filled to the said predetennined leve, i.e., full tank conditions, and is maintained at a pressure of 1,000 PSlA. Curve 56 does have a finite error between and 1,000 PSIA, but that error is quite small in the vicinity of point 57.

In FIG. 7, water density is indicated by a dotted curve 58 which begins at a point of 100 percent and zero pressure, and terminates at a point 59 of 74.4 percent at 1,000PSIA.

What is claimed is:

1. In a system wherein a liquid partially fills a tank and the tank has an internal pressure in excess of one atmosphere, the liquid having a density which decreases with the internal pressure and temperature, apparatus for producing a mechanical movement proportional to the liquid level in the tank, said apparatus comprising: a base; a differential pressure unit having one inletresponsive to the pressure above the liquid level and another inlet responsive to the pressure at the bottom of the tank, said unit having an output shaft rotatably mounted on said base, said output shaft having a position which is proportional to the difference between the pressures in said inlets; an output member whose position is proportional to the level of the liquid; a terminal shaft mounted on said base pivotally supporting said output member, said terminal shaft being mounted so as not to move in a direction perpendicular to its axis; a pressure sensor mounted on said base, said sensor having one end movable in response to said one inlet pressure; a first linkage fixed relative to and pivoted about the same axis as that of said output member; and means connected from said unit output shaft and from the said one end of said sensor to said first linkage to move said first linkage and said output member in accordance with the level of the liquid, said unit causing said means to move said output member in a predetermined direction when the inlet pressure differential increases and said one inlet pressure remains constant, said sensor causing said means to move said output member in a direction opposite to said predetermined direction when said one inlet pressure increases and said differential pressure remains constant.

2. The invention as defined in claim 1, wherein said means is constructed to cause said output member to indicate the accurate level of the liquid with zero error at a predetermined pressure in said one inlet substantially larger than one atmosphere and at a temperature which is the boiling point of the liquid at said predetermined pressure.

3. The invention as defined in claim 2, wherein a scale calibrated in liquid level is fixed relative to said base in a position contiguous to said output member.

4. The invention as defined in claim 3, wherein said means includes a second linkage fixed relative to and pivoted with said unit output shaft, a first auxiliary shaft mounted on said base, third and fourth linkages fixed relative to each other and pivoted on said first auxiliary shaft, a fifth linkage having one end pivoted on said second linkage in a position spaced from the axis of said unit output shaft, said fifth linkage having its other end pivoted on the end of said third linkage in a position spaced from the axis of said first auxiliary shaft, sixth and seventh linkages, said sixth linkage having one end pivoted to said fourth linkage in a position spaced from the axis of said first auxiliary shaft, said sixth linkage having its other end pivoted to one end of said seventh linkage, an eighthv linkage fixed relative to said output member, the other end of said seventh linkage being pivoted on said eighth linkage, a second auxiliary shaft mounted on said base, a disc rotatable on said second auxiliary shaft, a ninth linkage having one end pivoted on said sixth linkage between the pivot points of said fourth and seventh linkages thereon, said ninth linkage being pivoted on said disc at a point spaced from the axis of said second auxiliary shaft, rigid bracket means fixed to said one sensor end, a tenth linkage having one end pivoted on said bracket means and its other end pivoted on said disc in a position spaced from the axis of said second auxiliary shaft.

5. Theinvention as defined in claim 1, wherein a scale calibrated in liquid level is fixed relative to said base in a position contiguous to said output member.

I i t 

1. In a system wherein a liquid partially fills a tank and the tank has an internal pressure in excess of one atmosphere, the liquid having a density which decreases with the internal pressure and temperature, apparatus for producing a mechanical movement proportional to the liquid level in the tank, said apparatus comprising: a base; a differential pressure unit having one inlet responsive to the pressure above the liquid level and another inlet responsive to the pressure at the bottom of the tank, said unit having an output shaft rotatably mounted on said base, said output shaft having a position which is proportional to the difference between the pressures in said inlets; an output member whose position is proportional to the level of the liquid; a terminal shaft mounted on said base pivotally supporting said output member, said terminal shaft being mounted so as not to move in a direction perpendicular to its axis; a pressure sensor mounted on said base, said sensor having one end movable in response to said one inlet pressure; a first linkage fixed relative to and pivoted about the same axis as that of said output member; and means connected from said unit output shaft and from the said one end of said sensor to said first linkage to move said first linkage and said output member in accordance with the level of the liquid, said unit causing said means to move said output member in a predetermined direction when the inlet pressure differential increases and said one inlet pressure remains constant, said sensor causing said means to move said output member in a direction opposite to said predetermined direction when said one inlet pressure increases and said differential pressure remains constant.
 2. The invention as defined in claim 1, wherein said means is constructed to cause said output member to indicate the accurate level of the liquid with zero error at a predetermined pressure in said one inlet substantially larger than one atmosphere and at a temperature which is the boiling point of the liquid at said predetermined pressure.
 3. The invention as defined in claim 2, wherein a scale calibrated in liquid level is fixed relative to said base in a position contIguous to said output member.
 4. The invention as defined in claim 3, wherein said means includes a second linkage fixed relative to and pivoted with said unit output shaft, a first auxiliary shaft mounted on said base, third and fourth linkages fixed relative to each other and pivoted on said first auxiliary shaft, a fifth linkage having one end pivoted on said second linkage in a position spaced from the axis of said unit output shaft, said fifth linkage having its other end pivoted on the end of said third linkage in a position spaced from the axis of said first auxiliary shaft, sixth and seventh linkages, said sixth linkage having one end pivoted to said fourth linkage in a position spaced from the axis of said first auxiliary shaft, said sixth linkage having its other end pivoted to one end of said seventh linkage, an eighth linkage fixed relative to said output member, the other end of said seventh linkage being pivoted on said eighth linkage, a second auxiliary shaft mounted on said base, a disc rotatable on said second auxiliary shaft, a ninth linkage having one end pivoted on said sixth linkage between the pivot points of said fourth and seventh linkages thereon, said ninth linkage being pivoted on said disc at a point spaced from the axis of said second auxiliary shaft, rigid bracket means fixed to said one sensor end, a tenth linkage having one end pivoted on said bracket means and its other end pivoted on said disc in a position spaced from the axis of said second auxiliary shaft.
 5. The invention as defined in claim 1, wherein a scale calibrated in liquid level is fixed relative to said base in a position contiguous to said output member. 